Pulse Width Modulation, or shortly PWM, is largely used for controlling electric loads. FIG. 1 depicts a driving circuit, disclosed in the U.S. Pat. No. 5,917,720 in the name of the same applicant, of an inductive electric load, schematically represented by the resistor RL and the inductor LL connected in series, that uses a full-bridge power stage controlled by the pulse width modulation signals PwmA and PwmB. A sense amplifier SENSE AMPLIFIER senses the voltage drop on a sense resistor RS and generates a feedback signal that is subtracted by an error amplifier ERROR AMPLIFIER from an input signal Vin, and that represents the current to be forced through the electric load. The error signal Err is compared with two triangular ramp signals Tria1 and Tria2 for generating the pulse width modulation signals PwmA and PwmB.
Waveforms of the current Ivcm through the voice-coil motor and of the pulse width modulation signals PwmA and PwmB and of their difference Diff are shown in FIG. 2. The sign and the intensity of the current through the load Ivcm are controlled by adjusting the duty-cycle of the pulse width modulation signals PwmA and PwmB. When the duty-cycle of the signal PwmA is larger than 50% and the duty-cycle of the signal PwmB is smaller than 50%, the sign of the current Ivcm is as illustrated in the cited figure. The sign is inverted in the opposite case. The current is nullified by making the duty-cycles of the signals PwmA and PwmB equal to 50%.
This technique may be implemented with a sense amplifier SENSE AMPLIFIER for sensing the current flowing through the winding of the motor, having a high common mode rejection to the switching frequency in the whole range from ground voltage to the supply voltage.
Control circuits of a full-bridge power stage have been proposed in which the switches of a half-bridge are driven in PWM mode and the switches of the other half-bridge are either in a low or high saturation functioning condition, depending on the sign of the current to be forced through the electric load.
In the European patent application EP 1,641,115, herein incorporated by reference, a control system is proposed, the block diagram of which is represented in FIG. 3, that generates a logic driving signal of the switches of a half-bridge the value of which fixes the direction of the current through the load, and a drive PWM signal having a duty-cycle corresponding to the amplitude of the current to be forced through the load. The voltage difference at the inputs of the sense amplifier SENSE AMPLIFIER is referred either to a ground potential or to the supply voltage, thus it may not be indispensable to choose a sense amplifier having a high common mode rejection ratio in the whole voltage range from ground to the supply voltage, but it is sufficient that the common mode rejection ratio be sufficiently large for extreme voltages.
A detailed scheme of the control circuit of this prior document is depicted in FIG. 4. In order to reduce the absorbed power for low working currents, the control circuit is provided of linear amplifiers 5, 6 for driving the load in linear mode when the absolute value of the current is smaller than a pre-established threshold.
A characteristic of the system of FIG. 3 includes generating, with a comparator of the feedback loop, the logic driving signal that establishes the sign of the current through the load.
The U.S. Pat. No. 6,995,537 discloses a control circuit, depicted in FIG. 5, wherein the logic driving signal of the switches of a half-bridge that establishes the sign of the current is generated within the feedback loop. This circuit is adapted to control also low resistance electric loads (for example, about 5Ω) that may generate a relatively large back-electromotive force (for example, for voice-coil motors, of 5V or 6V). More particularly, it is effective also when the load absorbs small currents (for example smaller than 200/300 mA) and the back electromotive force is in phase with the forced current. Such a critical situation may occur, for example, while driving voice-coil motor for moving read/write heads of hard disks during seek operations, at the instant in which the current is inverted for braking the motor, that is moving at a relatively high speed.
A characteristic of the control system of FIG. 5 includes generating the logic driving signal that establishes the sign of the current, by an analog comparator for comparing the output of the error amplifier ERROR AMPLIFIER with a reference threshold.